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Necessity Condition in Implication ?

  1. Aug 27, 2014 #1
    P -> Q

    Code (Text):
    P   Q   P->Q
    T   T     T
    T   F     F
    F   T     T
    F   F     T
    I understand the condition "P is sufficient for Q". But I'm not getting the meaning of why "Q is necessary for P". What does this signifies, Please explain ..
  2. jcsd
  3. Aug 27, 2014 #2

    Stephen Tashi

    User Avatar
    Science Advisor

    Is this a question about interpreting English?

    One way to rephrase "Q is necessary for P" is to say "If Q is false then P must be false" or " not-Q implies not-P".

    Thinking of P and Q as events you could rephrase "Q is necessary for P" as "If P happened then Q must also have happened" or "If Q doesn't happen then P can't happen".
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